The invention relates to a shifting arrangement for two clutches for the shifting between a single-axle standard drive and a two-axle all-wheel drive for a motor vehicle having two drivable vehicle axles.
A known shifting arrangement of this type (U.S. Pat. No. 3,963,085), by using an additional brake for the braking of a central wheel and driving the planetary carrier by the engine in the case of a central differential gear, aims at an additional all-wheel operation with an unequal distribution of torques on the two driven vehicle axles as well as at an additional single-axle (rear-axle) drive with an overdrive effect. The control unit provides essentially a manual control by means of a manual shifting lever per se affecting the gear change box.
The objective on which the invention is based consists essentially of carrying out, in the case of a shifting arrangement of the initially mentioned type, the shifts as fast and smoothly as possible. But for driving stability, the shifting arrangement is able to provide, for the shifting from standard drive into the shifting step for a balanced all-wheel drive while driving, particularly while driving turns, an intermediate step during which a quasi balanced all-wheel drive is shifted. Namely, during the engaging of the clutch for the driving connection of one vehicle axle, the clutch, for the locking of the differential gear, must first remain engaged at the start of the shifting for a predetermined time period, during which the torque transmission of the other clutch is increased. However, under these circumstances, it must be ensured that the vehicle engine, after the expiration of the mentioned intermediate step, i.e., with the start of the disengaging of the clutch for the locking of the differential gear, cannot rev up to a high speed because of a lacking load.
The explained objective is advantageously achieved by providing hydraulic identifying valve that responds when both shift valves of the clutches are controlled by the control unit and as a function of the working pressure at least one of the control elements. The hydraulic valve interconnects the control elements such that the clutch torque of the clutch to be disengaged decreases only to the extent in which the clutch torque of the clutch to be engaged increases.
In the case of the shifting arrangement according to the invention, smooth and fast shifts are made possible because of the use of frictionally engaged clutches and pressure-medium-operated control elements. In the case of the shifting arrangement according to the invention, the shifting of the endeavored intermediate step can take place in such a way that with the use of two separate shift valves, the first shift valve for the first clutch for the locking of the central differential gear by means of the control unit is controlled in a delayed way with respect to the second shift valve assigned to one vehicle axle. Thus, initially the second vehicle axle absorbs some driving torque, and subsequently the torque decrease at the first clutch of the central differential gear, by means of the pressure-dependently controlled identifying valve, is rendered dependent on the torque increase of the second clutch of the connected vehicle axle.
The shifting arrangement according to the invention can operate irrespectively of whether the respective clutch is engaged by power assistance and disengaged by spring force, or is engaged by spring force and disengaged by power assistance.
For the hydraulic identifying valve that are used for providing a coupling (i.e. a mutual dependence) of the identifying torque of both clutches for permitting load shifts the arrangement, particularly in the case where both clutches are engaged by pressure-medium power assistance, can, for example, also be made in such a way that without the use of a hydraulic connection between the control elements in the working pressure line of at least one of the two control elements, a throttle valve with a variable throttle resistance is connected that is dependent on the working pressure of the other control element and responds to the joint control of both shift valves.
A large throttling restriction is connected between the second shift valve and the feed line and a smaller throttling restriction is connected between the first shift valve and the feed line. The smaller throttling restriction is connected with the second control element only when the first shift valve causes the disengagement of the first clutch. Because of this arrangement, the rise of the clutch moment of the second clutch for the driving connection of one vehicle axle in the intermediate step is relatively flat, but is steep during the transition into the connecting shifting step for the balanced all-wheel drive.
At least one throttling resistance is connected in series with the return line and the second control element when the second shift valve is switch into the switching step for standard drive. Thus, a sudden rise of the clutch moment of the first clutch of the central differential gear at the time of the shifting into standard operation, is reliably avoided.
At least one throttling resistance is provided in series with the first shift valve and the feed line and the return line for the operation of the shifting arrangement according to the invention for the shifts between the shifting step for a balanced all-wheel drive and an additional shifting step for a longitudinally locked all-wheel drive.
A pressure maintaining valve and flow limiting valve are connected in a compensating line with the pressure source or pumped and one of the two control elements to produce a permanent contact of the clutch disks also in the disengaged condition for fast shifts of the clutch for the connecting and disconnecting of one vehicle axle.
By making the first clutch engageable by spring force and the second clutch engageable by working pressure, it is ensured that, in the case of a breakdown of the pressure supply, a forced shifting into the standard drive takes place.
The identifying valve is preferably a check valve being controlled by the working pressure of the first control element to open against a prestressing force in the direction of second control element.
By providing the throttling resistance connected between the feed line and the first shift valve having a smaller cross sectional area than the throttling resistance connected between the feed line and the second shift valve, a simplification of the multiple function of a certain hydraulic resistor results.
By providing the check valve of the identifying valve connecting the first clutch with the second shift valve for the second clutch at a juncture of a throttling resistance connected to the second control element and the second shift valve, a safety circuit is formed. Thus, in the case of a breakdown of the first shift valve, a shifting into the standard drive results whenever the clutch of the central differential gear is disengaged.
A shiftable throttle resistance is connected between the first shift valve and the return line and switches, as a function of the working pressure of the first control element, between a small resistance value in case of a higher pressure and a larger resistance value in case of a lower pressure. Thus, shifting delays are avoided that are caused by an occurring clutch play in the clutch for the central differential gear.
A third clutch with a third pressure controlled control element and shifting valves allows for the shifting into a shifting step for all-wheel drive with a transversely locked driving axle.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.